1. Field of the Invention
This invention relates to a method and apparatus for producing fuel gas from cellulosic material. In particular, the invention relates to gasification of cellulosic material such as wood by pyrolysis in a rotary kiln.
2. Description of the Prior Art
A method and apparatus for making producer gas from carbonaceous material such as inferior grades of coal, wood waste or peat are disclosed in such prior art patents as U.S. Pat. No. 1,267,410 and 1,270,949 to Hornsey and U.S. Pat. No. 1,480,152 to Cox, wherein carbonaceous material is fed into the uphill end of an inclined rotary kiln; the carbonaceous material is repeatedly elevated by lifting buckets and showered downwardly as it is advanced through the kiln to dry the material and distill volatiles therefrom; air and/or steam is fed into the interior of the kiln through ports in the end walls and/or cylindrical walls to oxidize the carbonaceous material as it is being showered; and the resulting gases are withdrawn from one or both ends of the kiln. The carbonaceous material is diffused and distributed throughout the kiln by the lifting buckets as air is admitted from the ends of the kiln in such prior art apparatus with the result that oxygen and a substantial amount of solid particulates are entrained in the fuel gas, the characteristics of the fuel gas are inconsistent, and scrubbing equipment is required to clean the gas. The diffused carbonaceous material is oxidized throughout the kiln by the oxygen in the air or steam admitted into the kiln of such prior art apparatus with the result that the temperature of the carbonaceous material and carbon gases becomes so high due to the heat generated by the exothermic oxidizing reaction (C+O.sub.2 .fwdarw.CO.sub.2) that slagging occurs with consequent formation of "rings" of agglomerated material that impair transport of the material through the kiln and necessitate introduction of steam into the kiln to moderate the temperature rise. Due to the diffused scattered condition of the carbonaceous material, the oxygen in the admitted air or steam is not utilized efficiently in converting the carbon in the material to carbon gases, thereby necessitating an extremely long kiln in order to convert fully the carbon in the solid material to gas. Also, most prior art rotary kins, including those of the calcining, roasting and reducing types, are hundreds of feet in length and, consequently, have been expensive to construct.
Prior art patents such as U.S. Pat. No. 775,693 to Williams and U.S. Pat. No. 1,216,667 and U.S. Pat. No. 1,279,949 to Downs disclose admitting air under a bed of material being transported through a rotary kiln, but such prior art patents disclose injecting air under high pressure into the kiln to agitate and radially distribute the solid material, and consequently the above-discussed disadvantages would result if the apparatus of these prior art patents were used for the gasification of cellulosic material.